Lower generations of polyamidoamine (PAMAM) dendrimers were peripherally modified with anthracene moieties, and excimer emission from anthracene chromophores was investigated in an acetonitrilewater mixture at acidic and basic pH values. Results from fluorescence spectroscopic experiments suggest that 1) the propensity of anthracene-modified PAMAM dendrimers to aggregate in acetonitrile is substantial in the presence of 15-20 vol % of water, and 2) aggregate formation in anthracenemodified PAMAM dendrimers leads to unique morphologies in the ground state, where the anthracene units are pre-arranged to form stable excimers upon photoexcitation. Three types of anthracene excimers are generated in the system, with face-to-face, angular, and T-shaped geometry. The formation of different types of anthracene excimers was confirmed by steady-state and time-resolved fluorescence spectroscopic experiments. Experimental results further suggest that it is feasible to alter the type of excimer formed by anthracene units attached to the PAMAM dendrimers through altering the propensity for ground-state aggregation, Most excitingly, increased π conjugation in the molecular framework of anthracene-substituted PAMAM dendrimers leads to intense and exclusive excimer emission from anthracene at room temperature. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.